Memory cells in a semiconductor memory device such as an NAND flash memory are expected to be miniaturized in response to a request for a larger capacity and a lower cost per bit.
However, if the memory cells are miniaturized, the following kinds of degradation in characteristics cannot be avoided. First, when the memory cells are miniaturized, an increase in coupling between adjacent cells is a problem. Secondly, when the memory cells are miniaturized, variations in coupling ratio due to concave portions of control gates are a problem. Therefore, the miniaturization of the memory cells leads to a poorer performance of the NAND flash memory.